It is known that electrical equipment and devices, such as high voltage power transformers, are usually equipped with insulation systems based on cellulose materials and transformer oil. Insulation systems are used to insulate parts of the transformer connected to high potential from parts connected to other voltages or ground. Often the method of subdivision of oil volumes is used to increase the withstand strength of oil.
A main insulation problem at a core-type transformer is the problem of insulating high voltage windings from the core and from neighboring windings. In the winding insulation solutions used, a structure called a shield ring is often used to help deal with the stress on the corners of the windings which are made as cylindrical shells.
An example of prior art shield ring adapted for use with a high voltage power transformer will now briefly be described with reference to FIG. 1.
The winding end 1 of high voltage winding in a power transformer is shielded by an insulation structure consisting of pressboard barriers 2 which form a zig-zag pattern in surrounding transformer oil 3.
In the winding end 1, a shield ring 4 is used to increase the insulation on the corners of the winding more than is possible by adapting the insulation of current-carrying conductor of the winding itself. The shield ring 4 is built up from a core 5 and is circularly cylindrical. The outer layer of the core is covered with a conducting layer 6 which is potentially bound to the winding. The outer layer thus forms the electrode shape of the shield ring 4. Outside the electrode layer 6 of the shield ring 4 is a layer with solid insulation 7, preferably cellulose material. The layer 7 is thus facing the transformer oil 3.
The amount of solid insulation 7 material covering the conducting layer 6 on the core 5 of the shield ring 4 is exclusively homogeneously applied
The shield ring 4 has a few key properties. The most fundamental one is to insulate the corner of the winding, but the design of the shield ring also influence the oil flow that cools the winding, since the oil flows past the shield ring. Further, it transfers the spring force (vertical in figure) which is applied to the winding from the yokes to keep the winding firmly seated.
Prior art shield rings are constituted as so that the amount of solid insulation material 7 is homogeneously applied, which means that the mechanical, thermal and electrical properties of the shield ring are tightly bound together.